Corrosion inhibitor by means of 1, 2-dithiolanes



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CORROSION ITOR BY MEANS OF 1,2-DITHIOLANES No Drawing. Application August 1, 1957 Serial No. 675,578

6 Claims. (Cl. 252-395) This invention is concerned with the prevention of corrosion of metals, especially corrosion caused by acidic aqueous media. More particularly, the invention relates to a method for reducing the corrosive effect of an acidic composition in contact with an iron-containing metallic substance utilizing as corrosion inhibitors certain substituted 1,2-dithiolanes and their derivatives.

In the packaging of moist, wet or liquid materials in metal containers, a major concern is the prevention of corrosion of the metal with resultant contamination of the contents and destruction of the container. This is particularly true of packaged foods inasmuch as a large proportion of such foods are acidic, that is, have a pH below 7, and are highly corrosive to most metals.

For economic reasons, sheet steel is a desirable base from which to form containers and many means have been employed to reduce its corrosion by acidic media. Plating the steel with tin and coating either the steel or the tin-plated steel with resinuous organic coatings represent well-known methods for protecting the base metal of :a metal container. Such containers are satisfactory for many kinds of aqueous acidic products. However, certain acidic foods, particularly carbonated beverages, are

difiicult to preserve, even in tin-plated metal containers which have an additional resinuous topcoat. This is so because it is extremely diflicult to eliminate all pin holes from the various coatings, and corrosion of the base metal becomes a hazard. Consequently, means for preventing corrosion of metal containers by aqueous acidic contents are constantly being sought by workers in the container field.

It has now been found that the corrosive effect of an tent acidic composition in contact with an iron-containing metal can be reduced by a method which comprises including in said composition from 0.0001 to 10% by weight thereof of an organic compound of molecular weight not exceeding 500 and of the formula RR'COR, wherein R is a monovalent organic radical of the class consisting of atoms, and COR is selected from the class consisting of carboxyl and groups hydrolyzable thereto. The R" sub- ;stituent in the grouping -COR is preferably hydroxy, amido, lower alkoxy or an alkali metal such as sodium. IHowever, other metal salts, ammonium salts, N-substituted .amides and long-chain esters such as octadecyl can be used and fall within the scope of this invention. It will be understood that the term metal is used herein in its broadest sense and includes mixed metals or alloys as well as metals in the elementary sense.

This invention also contemplates the product produced in employing the above method. In other words, my invention is also directed to a manufacture comprising an iron-containing metal container having therein an acidic commodity and from 0.0001 to 10% by weight of the commodity of an organic compound of molecular weight not exceeding 500 and of the formula RRCOR", as defined above.

The iron-containing metals which are protected from corrosion by the present invention include cast iron, ingot iron, wrought iron, low carbon, medium carbon, and high carbon steels, and alloy steels containing 50% or more iron by weight. The examples which follow employ a. representative type of this group of iron-containing metals designated as black iron or black plate. This is a thin rolled sheet steel with an iron content above 98.5% which is widely used as a base for tin" can manufacture.

In a preferred embodiment of this invention, an acidic commodity, such as ginger ale or vinegar, is modified by the addition of 0.1% by weight of cc-llPOlC acid, i.e., 5-(1,2-dithiolan-3-yl)pentanoic acid. The resulting acidic commodity is packaged in a clean, uncoated, black plate steel can. The resulting manufacture shows no corrosion of the inner surface of the can when opened after several Weeks of storage at room temperature.

The following examples, in which parts are by weigh except where otherwise indicated, are intended for the purpose of illustrating the invention. It will be appreciated that the examples are not intended to limit the invention, since the invention is susceptible to modification without departure from the letter and spirit of the appended claims.

Example 1 A sample of commercial ginger ale (Canada Dry) is divided into two portions. Portion A is modified by the addition of 10 ppm. (parts-per million of dl-a-lipoic acid. Portion B is unmodified. The two portions are placed in separate glass containers. In each container is placed a 3" x 1" panel of black iron sheet degreased by vapor treatment with trichloroethylene immediately prior to immersion in the ginger ale. The containers are only partly filled, the upper portions containing air, and are capped lightly to prevent evaporation. After storage at room temperature for thirteen days, the metal panel in sample A is bright and the liquid only slightly discolored; whereas, the metal panel in sample B is discolored and the liquid is dark brown.

Example Black iron panels are washed with benzene, sanded, washed with acetone, and air dried prior to immersion in the following solutions. Solution C is a sample of commercial ginger ale (fCanada Dry) having added thereto p.p.m..of dl-a-lipoic acid, and solution D is a portion of the same ginger ale without the lipoic acid. The solutions and panels are stored in glass containers with vented caps. After 3 /2 months at room temperature, the metal in sample C retains its full brightness and the liquid shows no discoloration. In sample D, the metal is discolored and the liquid isdark brown.

Example 3 Three bottles of a commercial ginger ale (Canada Dry) are cooled to below 5 C. and opened in an atmosphere of nitrogen. To each of bottles E and F there is added-10 p.p.m. of dl-a-lipoic acid based on the ginger I alcpreseuu, Nomodifier is added to bottle G. In bottles E and G there are placed /2" x /2" panels of black iron V sheet prepared by sanding, followed by degreasing infeflllXiIlg trichloroethylene vapor. Into bottle F there is placed a similar black iron strip whichis degreased in trichloroethylene vapor without prior sanding. The bottles are sealed without access of air and stored at room temperature. After one month no change is observable in the metal or liquid in samples E and F; whereas in sample G both the metal and the liquid are discolored.

Example 4 Most of the lipoic acid dissolves in samples R and S. The slight excess is removed by filtration. The panels in the respective samples after three weeks and 4 /2 1 months at room temperature are as follows:

Metal Panel Sample 3 Weeks 4% Months Black coating Badg corroded.

Bright.

The liquids in samples R and S remain clear and colorless after 4 /2 months of storage.

Example 7 Six black iron panels are cleaned as in Example 2 and inserted respectively in thefollowing liquids in looselycapped glass containers in such fashion that about of each panel is submerged and about /5 remains in con tact with the air above the liquid. 7

Six 3" X 1" black iron panels are cleaned as in Example 1 2 and submerged respectively in the following liquids a pl Liquid in loosely-capped glass containers:

i 10% aqueous hydrochloric acid.

U 10% aqueous hydrochloric acid+0.l% 5-(2-methyl-1,3- Sample Liquid dithiane-4-yl)pentanoic acid.

V 10% aqueous hydrochloric acid+0.l% dl-a-lipoic acid. W 10% aqueous sulfuric acid. 1% aqueous hydrochloric-acid. X 10% aqueous sulfuric acid+0.l% 5-(2-methyl-L'3 1% aqueous sulfuric acid. dithiane-4-yl)pentanoic acid. Water. Y 10% aqueous sulfuric acid+0.l% dl-a-lipoic acid. 1% aqueous hydrochloric acid+0.1% dJ-a-lipoic acid.

1% aqueous sulfuric acid+0.1% dl-a-lipoic acid. water+0.l% dl-a-lipoic acid.

A small fraction of the lipoic acid remains undissolved in samples M and N and is removed by filtration before inserting the panels. After 15 days at room temperature the condition of the panels and the liquids in the respective samples is as follows:

Metal Panel Liquid Loose, brown coating cleazf) cloudy, sediment in bottom.

slighlgly cloudy, trace of ppt.

o Trace of corrosion clear.

After a total of five Weeks and five months, respectively, at room temperature the panels in the respective Sample 0 shows that at pH about'7 this invention provides'less corrosion resistance than at lower pH.

Example 6 Four 3 x 1" black iron panels are cleaned as in Example 2 and submerged respectively in the following liquids in loosely capped glass containers.

Sample Liquid 5% aqueous hydrochloric acid.

5% aqueous sulfnric'facid. 5% aqueous hydrochloric acid-{10.1% dl-a-lipoicacid.

- 5% aqueous sulfuric acid-{Q.1% dl-iz ll'poic acid,

All panels show corrosion in air above the liquids after one day. After one week and six weeks of storage at room temperature the submerged portions of panels in respective samples are as follows:

Metal Panels Sample One Week Six Weeks Partly dissolved Comlpletely dissolved (liquid clear,

ye ow Unchanged Unchanged (liquid slightly yellow). -do Unchanged (liquid colorless).

Black coating Clornlpletely dissolved (liquid clear,

ye ow A few dark spots Alien) corroded spots (liquid coloress Unchanged Unchanged (liquid colorless).

When the following compounds are substituted for the o -lipoic acid in Example 2, corrosion of black iron in contact with a commercial ginger ale is prevented:

5-(S-methyl-l,2-dithiolane-3-yl)pentanoic acid 6,8-dimercaptononanoic acid 5- Z-butyl-6-methyl- 1 ,3 -dithiane-4-yl) pentanoic acid 5-( 1,5 -dithiaspiro 5 .5 hendecane-2-yl)pentanoic acid 5 1,3-dithiane-4-yl) pentanamide V 5-(2,2-diphenyl-1,3-dithiane-4-yl)pentanoic acid Sodium salt of 5-(l,3-dithiane-4-yl)pentanoic acid 5-(2,2-dirnethyl-1,3-dithiane-4-yl)pentanoic acid Ethyl S-(Z-methyl-1,3-dithiane-4-yl)pentanoate 4-tert. butyl-5-(1,Z-dithiolane-B-yl)pentanoamidc 4-tert. butyl-6,S-dimercaptQoctanoarnide 2-( 1,2.-dithiolane-3-yl) acetic acid 3,5.-dimercaptopentanoic acid V 6-(1,2,dithiolane-3-yl)hexanoic acid 7,'9-dimercaptononanoic acid 9-( 1,2-dithiolane-3-yl) nonanoic acid 10,l2-dimerc aptododecanoic acid 4-ethyl-4-rnethyl-4-( 1,2-dithiolane-3-yl)butyric acid 4-ethyl-4-methyl-5,7-dimercaptoheptanoic acid 9-(5 butyl-1,2-dithiolane-3-yl) nonanoic acid 10,lZ-dimercaptohexadecanoic acid "The'above compounds may be used in this invention interchangeably "with their lower alkyl esters; that is,

methyl, propyl, butyl esters and the like; amides; salts, particularly their sodium, potassium and ammonium salts; and similar carboxylic derivatives which are readily hydrolyzed to the corresponding acids. Of this group, the free acids and the salts are preferred because of their greater solubility in aqueous systems.

Of the organic compounds useful for reducing corrosion according to the present invention, a preferred group is composed of u-lipoic acid, its reduction product, 6,8-dimercaptooctanoic acid, also known as gammalipoic acid, and the esters and salts of these acids. This group includes both the racemic forms of these compounds as well as their respective optical isomers when employed separately.

This preferred group of substituted 1,2-dithiolanes as well as the other organic compounds represented by the above formula R-R-COR", and their intermediates, can be prepared by one or more of the methods of assignees U.S. Patents 2,752,249, 2,752,373, 2,752,374, 2,792,406, and 2,801,261.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for reducing the corrosive effect of an acidic composition in contact with an iron-containing metal which comprises adding to said composition from 0.0001 to by weight of said composition, of an organic compound of molecular weight not exceeding 500 and of the formula where R is a monovalent organic radical of the class consisting of and the C-alkyl derivatives of these radicals wherein the alkyl groups are up to four carbons, R is a divalent hydrocarbon radical of up to 8 carbon atoms, R' and R"" are members of the group consisting of hydrogen and hydrocarbon radicals of 1-10 carbon atoms and taken together represents a divalent hydrocarbon radical, and COR is selected from the class consisting of carboxyl and groups hydrolyzable thereto.

2. The method of claim 1 wherein the organic compound is Ot-liPOiC acid.

3. The method of claim 1 wherein the organic compound is 6,8-dimercaptooctanoic acid.

4. An iron-containing metal container having therein an acidic commodity and, as an inhibitor of corrosion from 0.0001 to 10% by weight of the commodity of an organic compound of molecular weight not exceeding 500 and of the formula where R is a monovalent organic radical of the class consisting of References Cited in the file of this patent UNITED STATES PATENTS Searle Apr. 12, 1955 Latham et al June 26, 1956 

1. A METHOD FOR REDUCING THE CORROSIVE EFFECT OF AN ACIDIC COMPOSITION IN CONTACT WITH AN IRON-CONTAINING METAL WHICH COMPRISES ADDING TO SAID COMPOSITION FROM 0.0001 TO 10%, BY WEIGHT OF SAID COMPOSITION, OF AN ORGANIC COMPOUND OF MOLECULAR WEIGHT NOT EXCEEDING 500 AND OF THE FORMULA 